Well, I'm not sure I know what the true diffence is in cseg and xseg, but I'll give it a shot:

CSEG is the code segment and is where the Program CODE is located. XSEG is the Extra Segment and it is where the Program DATA is stored.

As far as where and how to declare variables. Depends on the language. For example in C:

unsigned char x = 0x0F;

In assy:

mov DPTR, #MYDATA

ORG 0300H
MYDATA: DB 0F


Anywho, hope that helps a little. Take a look at the tutorials on this webpage also and you will see how you "declare a variable".

Later,
Phil